The present invention relates to cathode ray tubes of the display or image type, and more particularly to an improved high brightness, high resolution screen which exhibits low noise level. The typical cathode ray tube user desires a screen having a high efficiency or high brightness of light output with high image resolution and low noise level. The screen noise level is measured as a function of the light output variation across a uniformly scanned line, and is thought to result from voids in the phosphor layer structure.
It has been the practice when high resolution low noise levels are required for the phosphor screen to use a cataphoretic deposition process for laying down the phosphor screen upon the tube faceplate. Such a cataphoretic deposition process is set forth in U.S. Pat. No. 3,525,679, owned by the assignee of the present invention. In such a cataphoretic deposition process, a conductive film is first deposited upon the faceplate and the finely divided phosphor electro-deposited upon this conductive layer from an electrolyte containing solution. Such electrophoretically deposited phosphor layers are typically very thin such as about 4 microns or several particle thicknesses thick. The conductive layer is thereafter removed from underneath the phosphor layer by treatment with a potassium cyanide solution and water flushing. Such electrophoretically deposited phosphor screens however have serious brightness limitations by way of comparison the brightness level of light output of a cataphoretic screen as compared to a more conventional gravity settled screen is about one-half the gravity screen brightness at a given screen current. The conventional gravity settled phosphor screens which have been used for some time in display and image cathode ray tubes while exhibiting high brightness levels is incapable of the high resolution and low noise level achievable with cataphoretically deposited screens.
It is desired to be able to achieve the highest possible screen brightness level with the highest possible resolution and low noise level characteristic for a cathode ray tube phosphor screen.